The pharmacological properties of some crustacean neuronal acetylcholine, gamma-aminobutyric acid, and L-glutamate responses.

1. A study was performed of the L-glutamate, gamma-aminobutyric acid (GABA), and acetylcholine (ACh) responses of cells in the stomatogastric ganglion of the crab, Cancer pagurus. 2. Ionophoretic or pressure application of L-glutamate revealed three classes of responses: a K+-dependent inhibition which reversed at 15-20 mV more negative than the resting potential; a Cl- dependent inhibitory response which was at equilibrium at the resting potential; and a depolarizing response. 3. Ionophoretic or pressure applications of GABA likewise produced three kinds of responses: an increase in K+ conductance, an increase in Cl- conductance, and a depolarizing response. 4. Picrotoxin (10(-6)-10(-5) M) was effective in blocking both the glutamate inhibitory responses. 10(-4) M-picrotoxin, which was necessary to produce a 50% block of the GABA-K+-dependent response, had no effect on the GABA-Cl- response. 5. beta-Guanidinopropionic acid (beta-GP) was found to be an agonist for the GABA-K+ response, but was ineffective in mimicking or blocking the GABA-Cl- response. 6. ACh applications produced large depolarizing responses with a pharmacological profile similar to that of the nicotinic ganglionic response in vertebrates. 7. The muscarinic agonist, acetyl-beta-methyl choline (MeCh), produced depolarizations which decreased in amplitude as the membrane was hyperpolarized from -40 to -100 mV. Pilocarpine and oxotremorine produced changes in the endogenous activity of ganglionic neurones. 8. Implications of these results for the identification of synaptic transmitters in the somatogastric ganglion are discussed.